Condensed Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]		Accumulated Other Comprehensive (Loss) Income [Member]		Treasury Stock [Member]
Beginning balance at Dec. 31, 2016	$ 505,209	$ 144,559	$ 230,494	$ 194,516		$ (19,454)		$ (44,906)
Beginning balance, shares at Dec. 31, 2016		26,589,353
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	33,830	$ 0	0	33,830		0		0
Other comprehensive loss, net of income tax benefit	3,111	0	0	0		3,111		0
Cash dividends declared ($0.60 per share)	(15,983)	0	0	(15,983)		0		0
Stock issued under dividend reinvestment and employee stock purchase plans	1,839	$ 0	130	0		0		1,709
Stock issued under dividend reinvestment and employee stock purchase plans, shares		63,683
Exercise of stock options	1,527	$ 0	(121)	0		0		1,648
Exercise of stock options, shares		84,870
Stock-based compensation	2,550	$ 0	2,550	0		0		0
Purchases of treasury stock	(3,285)	$ 0	0	0		0		(3,285)
Purchases of treasury stock, shares		(112,393)
Restricted stock awards granted, net of cancellations	0	$ 0	(881)	0		0		881
Restricted stock awards granted, net of cancellations, shares		45,823
Ending balance at Sep. 30, 2017	528,798	$ 144,559	232,172	212,363		(16,343)		(43,953)
Ending balance, shares at Sep. 30, 2017		26,671,336
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative Effect of New Accounting Pronouncement in Period of Adoption | Accounting Standards Update 2016-01 [Member]	0	$ 0	0	433	[1]	(433)	[1],[2]	0
Cumulative Effect of New Accounting Pronouncement in Period of Adoption | Accounting Standards Update 2018-02 [Member]	0	0	0	3,921	[1]	(3,921)	[1],[2]	0
Beginning balance at Dec. 31, 2017	603,374	$ 157,784	290,133	216,761		(17,771)		(43,533)
Beginning balance, shares at Dec. 31, 2017		29,334,859
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	32,171	$ 0	0	32,171		0		0
Other comprehensive loss, net of income tax benefit	(6,539)	0	0	0		(6,539)		0
Cash dividends declared ($0.60 per share)	(17,629)	0	0	(17,629)		0		0
Stock issued under dividend reinvestment and employee stock purchase plans	1,739	$ 0	140	1		0		1,598
Stock issued under dividend reinvestment and employee stock purchase plans, shares		62,271
Exercise of stock options	$ 1,131	$ 0	(43)	0		0		1,174
Exercise of stock options, shares	59,750	59,750
Stock-based compensation	$ 2,379	$ 0	2,379	0		0		0
Purchases of treasury stock	(2,384)	$ 0	0	0		0		(2,384)
Purchases of treasury stock, shares		(83,977)
Restricted stock awards granted, net of cancellations	0	$ 0	(617)	0		0		617
Restricted stock awards granted, net of cancellations, shares		34,173
Ending balance at Sep. 30, 2018	$ 614,242	$ 157,784	$ 291,992	$ 235,658		$ (28,664)		$ (42,528)
Ending balance, shares at Sep. 30, 2018		29,407,076

[1] (1) See Note 1, "Summary of Significant Accounting Policies - Accounting Pronouncements Adopted in 2018" for additional information.

[2] (1) See Note 1, "Summary of Significant Accounting Policies - Accounting Pronouncements Adopted in 2018" for additional information.